As known in the art, commercially available silicas can broadly be divided into two basic categories. These categories are those produced from a liquid phase and those from a vapor phase process.
Vapor process silicas, called fumed and pyrogenic silicas, are prepared by reacting silicon tetrachloride vapor with an oxygen-hydrogen gas at high temperatures to produce silicon hydrogen chloride. Pyrogenic silicas have high external surface areas and differ from other silicas (e.g., gels, precipitated silicas, etc.) prepared by the liquid phase process.
Liquid phase silicas include precipitated silicas produced by acidulating an alkali metal silicate with an acid such as sulfuric acid. Liquid phase silicas also include silica gels and colloidal silicas.
In recent years, and to further expand the utility of precipitated silicas, prior art workers have developed new techniques for producing precipitated silicas having new and unique properties.
U.S. Pat. No. 3,967,563 discloses a process for precipitating a very high structure silica. In the disclosed process, the rate of acidulation is very carefully controlled, particularly at the outset of the reaction and at the point at which precipitation initially occurs. The silica product produced by this method has a very high structure, a wet cake moisture between 86-89%, an oil absorption of about 220-250 cc/100 g and a surface area of about 250-280 m.sup.2 /g.
U.S. Pat. No. 4,251,281 discloses a process for producing synthetic amorphous silica which has a surface area of between 80-310 m.sup.2 /g and an oil absorption index of between 240-320 cc/100 g. The process involves reacting a strong acid with an alkali metal silicate solution wherein the acid is added to the solution at a variable rate to maintain a residual alkalinity of the medium substantially constant with time.
U.S. Pat. No. 5,234,673, a prior patent of the present inventors, discloses a process for producing a precipitated silica which has a high structure, and an oil absorption of less than 300 cc/100 g, the silica being produced by acidulation of alkali metal silicate.
The present invention provides a novel silica product which is a high structure product with smaller pores than known previously, which silica is useful in paints, as a conditioning agent in foods, and in dentifrices, and is produced by acidulation of an alkali metal silicate under novel reaction conditions.